About 40 years ago main industrial ion sources were of so-called gridded types which are described in detail by H. Kaufman and R. Robinson in “Operation of Broad-Beam Sources”, Commonwealth Scientific Corporation, Alexandria, Va., 1987. These gridded ion sources are utilized for development of ion beams of low and medium energy (100-1500 eV) in varieties of thin film technologies for cleaning, etching, sputtering, deposition. However, despite of continued utilization of gridded ion sources at present time, they are not used broadly as it was earlier. That is because of their complexity, high cost and problems with an ion beam neutralization that prevent obtaining high ion beam currents at low energies of about and especially under 100 eV. The other type of ion sources are the Hall-current ion sources that are utilized for about last 30 years for development of ion beams of low and medium energy (20-500 eV) in varieties of thin film technologies for cleaning, sputtering, deposition, ion assist, etc. Hall-current ion sources sometime also are called as gridless ion sources in comparison with gridded ion sources. Hall-current ion sources are substantially simpler than gridded ones and they have no problem with ion beam neutralization caused by a space charge limitation that takes place in the gridded ion sources. Practically all ion sources at the beginning have been developed from the electric propulsion technologies utilized very successfully for propulsion of space satellites and other space apparatuses.
One of the most frequently utilized Hall-current ion sources in thin film technology is a so-called end-Hall ion source. This gridless ion source has discharge chamber occupied by a massive hollow anode in a form of a cut cone. Magnetic field in such ion source is provided by a magnetic system that consists usually of a permanent magnet and an ion source's external shell made of a magnetically soft iron. Permanent magnet fabricated in a form of a cylinder of certain length of 40-100 mm with a diameter of 10-20 mm and has at its end pieces-poles a magnetic field value from about 400-500 G and to about 1000-1500 G. In some cases, instead of a permanent magnet a solenoid made of a heat-resistant wire provides necessary magnetic field in a discharge channel. Solenoid has advantage that its magnetic field can be regulated by a direct current. However, a solenoid needs additional power supply for that current.
Here is how an end-Hall-current ion source operates. Working gas such as Argon, Xenon, Oxygen, Nitrogen, Hydrogen, Methane and others is introduced in to a discharge channel through a series of holes in lower part of a discharge channel, under a hollow conical anode, or from a side wall under anode, or other parts of a discharge chamber. At the same time, a positive potential is applied to anode and there is realized a certain potential drop between anode and cathode. A cathode placed usually outside of an ion source's exit flange produces electrons which travel in to a discharge channel and start discharge at a discharge ignition voltage, when the condition for beginning discharge takes place, as it is determined by the Paschen conditions (Paschen curves), which is a product of pressure p and distance d between electrodes (anode and cathode) at low pressures (Vign=f(p·d)). After achieving of an electric breakdown (also called ignition) state determined by the Paschen curve for a particular working gas, an electrical discharge begins. This discharge produces ions that move into a cathode side together with neutralizing electrons supplied by cathode. These ions accompanied with electrons in a form of ion beam possessing certain energy are utilized for bombardment of targets, or substrates. Bombarded targets or substrates can be either sputtered, or influenced by an ion beam depending on ion beam energy and current. Sputtered particles move out of target to a substrate and make thin film depositions of a designed structure. If an ion beam is utilized for influence of deposited thin film for improvement of the thin film properties it is called as an ion assisted deposition. Despite that in a scientific literature all estimations are made for an ion beam, all ion beams, as a rule, are always accompanied with electrons that are supplied by cathode. Ion beams without electrons is rare phenomenon; in such a case, they become unstable and expand due to mutual repulsion of ions.
In Hall-current ion sources electrons, as it was above mentioned, are needed for ionization of a working gas and for ion beam neutralization. Such electrons are usually supplied by a cathode of one or another type. Also for correct organization of electric discharge processes in an ion source discharge channel it is desirable to avoid straight propagation of electrons from cathode to anode and to prevent arcing during discharge. Arcing in ion sources is prevented by a magnetic field that is applied in to a discharge channel; in such a case electrons become “magnetized”; theirs direction of propagation changes by a crossed magnetic field component; an electron velocity component that is parallel to a magnetic field makes no impact on electron's rotation. It is also preferable that in a discharge channel should be existed a substantial transversal component of a magnetic field. Such magnetic field usually, as above mentioned, is provided either by a permanent magnet or magnets or electromagnetic coil, or coils. Whole design of an ion source includes a magnetic circuit with a permanent magnet, or electromagnetic coils and an ion source's chamber parts usually made of a magnetically soft steel, except anode and a gas distributing system. Though there are anodes made of magnetic material in a so-called Anode Layer Closed Drift thruster-ion source. In a magnetic circuit, there are observed two poles placed in a discharge chamber. They are usually a permanent magnet's top (first pole) and an ion source's exit flange (second pole).
Two types of Hall-current ion sources are on a market for industrial applications. One of the first ion sources of such type was a Closed Drift ion source, as described by V. V. Zhurin et al in article “Physics of Closed Drift Thrusters” in Plasma Sources Science & Technology, Vol. 8 (1999), beginning on page R1. The most well-known variation of another type of a Hall-current is an end-Hall ion source as it was described in U.S. Pat. No. 4,862,032 by H. Kaufman and R. Robinson “End-Hall Ion Source”. Also, end-Hall ion sources of various designs were developed by: W. G. Sainty, “Ion Source”, U.S. Pat. No. 6,849,854, Feb. 1, 2005; V. V. Zhurin, “Hall-Current Ion Source for Ion Beams of Low and High Energy for Technological Applications”, U.S. Pat. No. 7,312,579, Dec. 25, 2007; D. M. Burtner, S. A. Townsend, D. E. Siegfried, V. V. Zhurin, “Fluid-Cooled Ion Source”, U.S. Pat. No. 7,342,236, Mar. 11, 2008. End-Hall ion sources have been described in many patents and articles mainly by major producers of ion sources in USA and Russia. In the last 10-15 years new producers of industrial ion sources were appeared in China and South Korea as it was described by V. V. Zhurin in “Industrial Gridless Broad Beam Ion Sources and the Need for Their Standardization. Part 2. End-Hall Ion Sources for Thin Film Technology”, in Vacuum Technology & Coating, May 2009, p 40-51.
The main and most important operating parameters of ion sources are: 1. discharge current Id and its operation range; 2. discharge voltage Vd and its operation range; 3. stability of discharge current and voltage in broad range of these values. Actually, a discharge current Id is responsible for development of an ion beam current I; that is a number of ionized particles per unit time and area. A discharge voltage Vd is responsible for an ion beam energy Ei, which is kinetic energy of ionized particles moving with high velocity in discharge channel. In a performance for the main operational characteristics such as an ion beam current an ion beam mean energy Ei and purity of an ion beam (presence of varieties of contaminants from sputtered parts of ion source, or from inadequate pumping of a vacuum chamber and ion source) of a Closed Drift type ion source substantially exceeds an end-Hall type. However, end-Hall ion sources, in general, have a simpler design than a Closed Drift type. Also, unfortunately for regular users, Closed Drift ion sources need to be optimized by a magnetic field and such optimization does not have simple dependence of a magnetic field value as described by V. V. Zhurin et al in article “Physics of Closed Drift Thrusters” in Plasma Sources Science & Technology, Vol. 8 (1999), beginning on page R1. Closed Drift ion sources as Electric Propulsion thrusters were utilized successfully on many space satellites.
End-Hall ion sources usually have a range of discharge voltages from about 50-60 V with noble gases, and up to 80-100 V with Oxygen and Nitrogen, and up to about 300 V and can be used for obtaining low energy ion beams with ion beam mean energies of 30-40 eV and up to about 180-200 eV. Closed Drift ion sources can not deliver low energy ion beams like end-Halls. Closed Drift ion sources usually start operation at discharge voltages of about 80-100 V with noble gases and at 100-120 V with Oxygen and Nitrogen and their maximum discharge voltages can be easy extended up to about 1000 V. This is explained by a presence of a substantial value of a transversal magnetic field component in Closed Drift ion sources in comparison with end-Hall ion sources that have a quite low value of a magnetic field transversal component. End-Hall ion sources have mainly a longitudinal component of magnetic field. That is why end-Hall ion sources have easy ignition discharge conditions at comparatively low discharge voltages of 50-60 V and Closed Drift ion sources due to a presence of a substantial transversal component of magnetic field experience problems in ignition at low discharge voltages. Also Closed Drift ion Sources in general have a so-called positive magnetic field gradient in a discharge channel as it was described by V. V. Zhurin et al in article “Physics of Closed Drift Thrusters” in Plasma Sources Science & Technology, Vol. 8 (1999), beginning on page R1. A positive magnetic field gradient in a discharge channel, when magnetic field increases from anode to a discharge channel exit, allows suppressing many different types of oscillations and operating with discharge voltages up to 1000 V. An end-Hall type ion source has a negative magnetic field gradient, when magnetic field decreases from anode to a discharge channel exit, and, because of this, has problems operating at discharge voltages over 300 V.
The main features of end-Hall ion sources suffer from the following shortcomings. An ion beam current, which is a derivative of a discharge current, is usually a small portion of a discharge current; in other words, it means that a working gas is not sufficiently ionized and substantial portion of working gas leaves a discharge channel not ionized. In one of the varieties of Hall-current ion sources that is called as a Closed Drift Ion Source, which is described in article “Physics of Closed Drift Thrusters” in Plasma Sources Science and Technology by V. V. Zhurin et al, and in U.S. Pat. No. 7,312,579 by V. V. Zhurin, it is shown that the Closed Drift ion sources have a high ratio of an ion beam current Ii to a discharge current Id, or Ii/Id≈0.8-0.9. However, in the end-Hall ion sources that utilized in thin film technology more frequently than Closed Drift ion sources, the ratio of an ion beam current Ii to a discharge current Id, is quite low, or Ii/Id≈0.2-0.25. It means that the end-Hall ion sources, in order to produce the same effect by an ion beam on a target or a substrate have to apply more electric power in to an end-Hall ion source discharge channel than a Closed Drift ion source.
Another variety of ion sources utilized in thin film technology, and that was above mentioned, is called the gridded ion sources that considered in general as electrostatic particles acceleration, in which after discharge in a discharge chamber the ionized ions are extracted through a system of screen and accelerating grids with aligned small apertures in the grids. An ion beam is developed from numerous individual beamlets when they leave the accelerator's grid. Gridded ion sources operate successfully at relatively high discharge voltages from about 200 V and to about 1500 V. Ion beam currents are not high, especially at lower discharge voltages, they are about 100-200 mA. However, the gridded ion sources have comparatively monoenergetic ion beam energy distribution and high translation of applied potential into an ion beam energy.
An ion beam of end-Hall ion source in comparison with gridded ion sources has no monochromatic energy, instead it has quite a broad energy distribution, and in practice it is usually determined through a mean ion beam energy Ei which is a total ion energy distribution divided by an ion beam current. In all Hall-current ion sources that utilize a source of electrons for ion beam neutralization a mean ion beam energy Ei ratio to an applied electric potential Vd multiplied by an electric charge is also a part of this applied potential, or Ei/eVd≈0.6-0.7. For other types of ion sources, like linear Anode Layer ion sources, which are a part of Closed Drift family ion sources, that operate at comparatively high discharge voltages of about 500-4000 V and without an external source of electrons for ion beam neutralization the ratio of a mean ion beam energy Ei to an applied electric potential Vd multiplied by an electron charge can be as Ei/eVd≈0.5 at maximum, but, in general, this ratio is 0.1-0.2.
An end-Hall ion source can provide comparatively high ion beam currents over 1-2 A at low ion beam mean energies of around 100-150 eV with quite broad ion beam energy distribution, for example, Ei≈125±50-75 eV at discharge voltage Vd=210 V. Gridded ion sources can not provide high ion beam currents at low energies. They can deliver about 100-200 mA at low energies, but gridded ion sources can deliver quite a monochromatic ion beam energy, for example, Ei≈575±25 eV at accelerating voltage of Va=600 V. For many thin film deposition tasks it is necessary to have high ion beam currents and ion beam energy in a narrow range of values. In such a case, it is simpler to design required conditions for interaction of ion beam of certain energy with target and/or substrate. This means that it will be desirable to develop a Hall current ion source with a high ion beam current with comparatively monochromatic ion beam energy similar to a gridded ion source. Also, when end-Hall sources are utilized in an ion assisted deposition technique as additional source of ions for enhanced impact-processing of a sputtered thin film, in certain cases, end-Hall ion beams are inadequate for a continuous stress modulation through the entire deposited thin film layer, simply because of a presence of very wide range of ions with various energies. For example, if an ion assisted energy is about 100 eV, but it distribution has a spread of over 50-100 eV, these energetic ions instead of compacting a deposition can destroy, remove some parts of deposition having energy higher than a sputtering threshold of deposited material, and low energy ions would not produce a desirable effect of a thin film deposition compacting, or will be lost in the process. In other words, such a broad beam energy distribution could be harmful, or inefficient for certain thin film deposition processes.
In many cases, especially at high discharge currents, an ion beam coming out of an end-Hall ion source's discharge channel is quite contaminated by materials of a discharge channel: an anode, a gas-distributor, called frequently as a reflector, an external pole-exit flange, and by a Hot Filament, or a Hollow Cathode materials utilized for an ion beam neutralization. Due to these problems, it also will be desirable to design an end-Hall ion source that produces substantially less contamination of an ion beam leaving an ion source discharge channel in to a target and a substrate's side.
Another very important feature of Hall current ion sources is necessity of stable, reliable operation of such ion sources with reactive gases. During operation with reactive gases there are developed oxidized, nitridized particles that deposit on a discharge channel's walls and, in particular, on anode surface and gradually change its electrical conductivity. This process called sometime as anode “poisoning” leads to a situation when a discharge voltage at a constant discharge current provided by a Power Supply due to loss of anode's surface electrical conductivity starts gradually increasing. With a constant discharge voltage provided by a Power Supply a discharge current decreases gradually. In both cases the operating conditions drastically change and usually cause unplanned interruption of a process. This anode “poisoning” state is very serious and lead to development of several patent applications, in which various methods how to reduce the anode “poisoning” were introduced. One of them a U.S. Pat. No. 6,750,600 “Hall-Current Ion Source” by H. R. Kaufman, J. R. Kahn, R. S. Robinson, V. V. Zhurin suggests utilization of a grooved anode in the end-Hall type ion source with some parts of anode surfaces that do not “see” returned back dielectric and insulating particles, because in pressure conditions that end-Hall ion source operates particles propagate along straight lines. Also in this patent there is suggested a placement of a shield in front of a discharge channel to reduce a returned back flow of dielectric and insulating particles from a target and a vacuum chamber into an anode surface.
In a U.S. Pat. No. 7,312,579 “Hall-Current Ion Source for Ion Beams of Low and High Energy for Technological Applications” by V. V. Zhurin a working gas is introduced through holes in anode as an alternative way for gas application. In this patent an increased area of a gas distributing system under anode and in this area is suggested with numerous holes for a gas application directed at a certain angle to a source's axis for better gas distribution under anode. Also in this patent it is suggested to utilize an electron emission current exceeding a discharge current in the discharge operation mode of a so-called non-self-sustained regime. All these measures helped to improve an ionization process reducing an ion beam energy at low discharge voltages and correspondingly to an ion beam energy distribution.
Another big problem that exists with appearance of industrial ion sources is their adequate neutralization of a positively charged ion beam. For this purpose there are utilized various sources of electrons such as a Hot Filament, a Hollow Cathode, a Plasma Bridge and many other ways producing electrons. These electron sources were recently described in detail by V. V. Zhurin in “Cathodes-Neutralizers for Ion Sources, Part 1, Introduction, Hot Filaments”, Vacuum Technology & Coating, January 2010, p 45-53, “Cathodes-Neutralizers for Ion Sources, Part 2, Hollow Cathodes”, Vacuum Technology & Coating, February 2010 p 32-44, “Cathodes-Neutralizers for Ion Sources, Part 3, Plasma Bridge, RF and Other Neutralization Methods”, Vacuum Technology & Coating, May 2010, p 38-47. The most simple and inexpensive cathode neutralizer is a Hot Filament that heated to high temperatures over 2000-2500 K providing a thermoelectron emission. Hot Filament is cheap and can be easily changed after each vacuum chamber opening, if necessary. Its filament, usually made of Tungsten wire, is placed on a way of an ion beam and during its operation it is bombarded by ion beam, becomes sputtered by ion beam and breaks under its impact. At the same time, an ion beam becomes contaminated by Tungsten particles. For pure thin film processes such contamination is undesirable. Hot Filaments usually last not long, from several hours to just over 10 hours at moderate applied powers of Id=1-5 A and Vd=100-150 V. The higher the discharge current and voltage, the shorter a Hot Filament lifetime. The best ion beam source neutralizer is a Hollow Cathode, but it is quite complex in operation and maintenance, and expensive. That is why most ion sources users prefer Hot Filaments. Unfortunately, a Hot Filament consumes a quite high electric power and radiates into practically all directions and into a side of a target, and a substrate heating them that in many cases is very undesirable, especially, for the temperature sensitive materials. All these problems were discussed in detail in above mentioned articles by V. V. Zhurin published in Vacuum Technology & Coating. To find a more optimum way of utilization of a Hot Filament without ion beam contamination and with longer lifetime would improve many thin film deposition operation techniques significantly.